1. Field of the Invention
The present invention relates to using the Data Over Cable Service Interface Specification (DOCSIS) to identify characteristics of a satellite communications system. More specifically, the DOCSIS specification can be used to indicate a message collision in the upstream channel of a satellite communications system.
2. Description of the Related Art
In DOCSIS related broadband data communications architectures, data is transferred between a central location and many remote subscribers. Terms for the centrally located equipment include Headend (cable systems), Wireless Access Termination System (WATS—broadband terrestrial fixed wireless), or Satellite Gateway (two-way satellite). Terms for the subscriber equipment include Cable Modem (CM—cable systems), Wireless Modem (WM—broadband terrestrial fixed wireless) or Satellite Modem (SM—two-way satellite). For the remainder of this document, the satellite terminology will be used in the description of this invention. The communication path from the Gateway to the SM is called the downstream. The communication path from the SM back to the Gateway is called the upstream. FIG. 1 illustrates a high level diagram.
In standard DOCSIS based systems, traffic for many SMs is placed in Time Domain Multiplex (TDM) fashion on a downstream channel. SMs also share upstream channels in a Time Domain Multiple Access (TDMA) fashion. If a SM is using a given downstream, its upstream traffic is assigned to one of typically several upstream channels that are associated with each downstream. The set of the downstream channel and the corresponding upstream channels is sometimes called a MAC Domain.
In order to allow SMs to share upstream channels, the Gateway sends bandwidth grant map messages to the SMs for each upstream in the MAC domain of the downstream. In the DOCSIS protocol, these messages are referred to as MAP messages. The MAP messages identify burst types, give start times, duration's for each SM that is granted bandwidth on a given upstream channel. The MAP messages also identify time interval for initial ranging, for SMs that wish to attempt to register for the first time on the channel, and multicast Request Access windows, for SMs that wish to make bandwidth requests.
Both initial maintenance bursts, and multicast request access regions are contention type requests. This means that multiple users may attempt to utilize a given burst time simultaneously. If this occurs, there is a “collision” and the burst from both SMs are lost. The DOCSIS protocol has a algorithm by which the SMs recognize that a collision has occurred and retry their burst at random back-off intervals. When a given upstream channel is very congested, there is a significant probability that a SM's burst might suffer from several collisions before it gets through successfully.
In satellite channels, a collision event is more detrimental to system performance due to the long delays involved. What is needed is a mechanism to signal to SMs whether congestion exists on a given upstream so that collision events can be reduced during initial ranging and when multicast bandwidth requests are used.